This invention relates to a focus error detection apparatus and more in detail to a focus error detection apparatus suitable for a light beam application apparatus, such as optical video disk player, optical audio disk player, or optical digital recording apparatus, in which the focusing position of the illuminating light must follow variations in position of the reflective surface.
In an optical disk apparatus in which a surface of a rotating recording medium is illuminated by laser light and information is optically recorded, reproduced or erased, an auto-focusing servo-system is necessary, which moves an objective lens in the optical head according to the movement of the reflective surface in the optical axis direction taking place in the optical disk, which is rotating, so that the data recording surface is always within the focus depth of the laser spot. The auto-focusing servo-system consists of a servo-motor, e.g. of voice coil type, for moving the objective lens in the optical axis direction, focus error detection optics, and a servo-amplifier for actuating the servo-motor according to focus error thus detected. However, among these elements the focus error detection optics are specifically important and in the case where it is applied to an optical disk apparatus, it is desirable to adopt a construction, in which variations of reflected light due to information pits on the information recording surface, pre-groves forming tracks, etc. don't influence focus error signals. Further, in order to obtain correct focusing control, a device construction is desired, in which, even if displacement in optical axis of the reflected light coming from the information recording surface is produced, e.g., by displacement in position of the optical system, it doesn't influence the focus error signals.
Heretofore, various methods have been proposed for constructing a device for detecting the focus error described above, in one of which the reflected light coming from the information recording surface (reflecting surface) is focused by a lens and a knife-edge is disposed at the convergence point of the light so that only a part of the reflected light reaches a photo-detector located behind the knife-edge (e.g. U.S. Pat. No. 4,450,547). According to this method, it is possible to obtain a semi-circular optical image on the photo-detector, which rotates according to the magnitude of the focus error, by disposing a cylindrical lens between the focusing lens and the knife-edge and to obtain a high precision focus error signal by means of a two-divided photo-detector by using its differential output. However, according to this method, since the focus error signal depends on the relative positional relation between the knife edge and the focused light, there is a problem that variations in position of the knife edge due to thermal expansion and deviations of the optical axis of the reflected light influence the focus error signal. Further, according to another method, as described, e.g., in Japanese Patent unexamined publication No. 84-77637, an optical element for separating the central portion and the peripheral one of the light beam into different directions is disposed on the optical path along which the reflected light is focused and the light beams thus separated are received by separate photo-detectors so that a focus error signal is obtained by using the difference between their outputs. However, since variations in light intensity produced by the information pits and the tracks stated above appear differently for the central and peripheral portions, there remains a problem in this method that noise components are contained in he error signal.
In addition, for an optical disk device using such a focus error detection apparatus, since the information reproduction optical system and the optical system for the focus error detection and the track error detection are separated, there is a problem that the optical head is too big and includes too many parts and that as the results it is too expensive.